The production of ethylene requires a number of process steps through which any of a variety of hydrocarbon feeds can be refined to generate various products including ethylene. The predominate process for producing ethylene is steam cracking. According to this process, hydrocarbon feed is heated in a cracking furnace and in the presence of steam to high temperatures. The resulting products leave the furnace for further downstream processing.
Once the desired conversion of feed has been achieved, the process gas must be rapidly cooled, or quenched, to minimize undesirable continuing reactions that are known to reduce selectivity to ethylene. The vast majority of ethylene furnaces currently in use employ so-called “transfer line exchangers” (TLE). These devices are heat exchangers that rapidly cool the process gas by generating steam. The resulting steam is typically generated at high pressures (e.g. 600-2000 psig).
Many of the transfer line exchangers in service employ a double pipe or double tube construction with the high temperature cracking furnace effluent introduced into the interior pipe and a cooling medium such as water being introduced into the annular space between the two tubes. Double pipe exchangers may be configured as bundles or as so-called “linear” units. The advantage of the linear type unit is that the adiabatic time between the furnace outlet and the cooling tube inlet can be minimized to allow an enhanced ethylene selectivity. Linear units also benefit from the lack of a tubesheet area which would otherwise be exposed to the hot process gas and are thus subject to various mechanical and erosion concerns. Further, in linear units, the process flow is more evenly distributed among the cooling tubes, with no turbulence and recirculation in the inlet chamber that causes coking and polymerization of the valuable cracking products before entering the cooling tubes.
Steam generating transfer line exchangers have found particular utility in the initial quenching of effluent produced in furnaces cracking naphtha and lighter feeds. In liquid cracking furnaces processing heavy gas oil feeds, direct injection quench points are often required because of the rapid fouling that occurs in the TLE cooling tubes when the cracked gas is cooled below the dew point of the heavy ends of the cracked gas.
As may be appreciated, when gas or liquid feeds are cracked, high boiling point molecules are formed. A portion of these molecules are trapped on the radiant tube wall of the furnace where they polymerize to coke. Molecules not trapped enter the transfer line where they polymerize to form heavy, high boiling point asphaltene-type coke precursor molecules. When the cracked gas is cooled, these high boiling point coke precursor molecules condense and form a viscous liquid layer on the TLE cooling tube walls. The high velocity process gas in the cooling tube may sweep much of the liquid away, but some of it will be trapped on the cooling tube walls where it eventually will harden and turn to coke. The amount of coke formed on the cooling tube walls is a function of several factors: the severity of the cracking, the unfired residence time, the final boiling point of the heaviest molecules in the feed, the temperature to which the cracked gas is cooled in the transfer line exchanger, and the temperature of the transfer line exchanger cooling tube walls.
When the cracked gas traverses through the transfer line exchanger cooling tube, more of the heavy molecules contained therein polymerize to coke precursors as they are cooled to lower temperatures. As they proceed along the transfer line exchanger cooling tube, the amount of liquid and heavy molecules condensed on the tube wall increases as the temperature decreases, the viscosity of the condensed liquid increases and the condensed liquid is more readily trapped on the cooling tube walls. As a result, long transfer line exchangers that cool the cracked gas to low temperatures will coke more than shorter transfer line exchangers which do not cool the cracked gas to the same degree. Thus, for heavy feeds, short exchangers that cool the cracked gas to only about 950° F. (510° C.) are preferred.
In order to achieve best selectivity to ethylene, it is necessary to minimize both the residence time (“fired time”) and the adiabatic time (“unfired residence time”) within an ethylene furnace. The latter time refers to the amount of time required for the process effluent to pass from the fired zone of the furnace to the entrance of the TLE. One set of existing solutions that have been developed to minimize adiabatic time are the so called close-couple type transfer line exchangers. According to this design, the quench exchanger tubes are connected directly to the furnace effluent tubes without intermediate manifolding.
As indicated, the temperature of the wall of the transfer line exchanger cooling tube influences the amount of liquid condensed and the amount of coke formed in the TLE cooling tube. As may be appreciated, low temperature cooling tube walls coke more readily than high temperature walls. Therefore, transfer line exchangers designed for heavy feeds must generate high pressure (1500 psig) steam, while exchangers that cool the light gas feed generate medium pressure (600 psig) steam. Moreover, the higher the cracked gas velocity in the cooling tube, the thinner the liquid layer and the lower the amount of liquid that will be trapped on the cooling tube wall.
In view of these factors, close-coupled transfer line exchangers, even medium pressure (600 psig) steam generating transfer line exchangers, are frequently designed as double-pipe units. Advantageously, the close coupled design concept enables the unfired outlet time to quench to be shorter, thus enhancing selectivity. Additionally, separation in the unfired outlet zone can be minimized, thus minimizing coking between the fired zone and the TLE, avoiding conventional circular TLE inlet head coking, which can obstruct TLE tubes when spalled. Further advantages include the avoidance of conventional circular TLE inlet tubesheet coking, which can obstruct TLE tubes when spalled, the elimination of TLE inlet tubesheet erosion problems, and the enablement of faster and more effective decoking of the TLE. Each close-coupled TLE tube is fed either by a single radiant tube or dual radiant tubes.
Ethylene furnaces are typically used for the production of a wide variety of products. These include hydrogen at the light end to steam-cracked tar at the heavy end. As a general matter, the heavier the feedstock, the greater the yield of steam-cracked tar. In naphtha crackers, the effluent composition contains a tar content that is high enough that the heaviest components will commence condensing if cooled to approximately 600° F. (315° C.). As feedstocks get heavier, the tar yield rises and the temperature at which condensation commences also rises. Should condensation of the effluent occur in the transfer line exchanger, heat transfer is substantially impeded and a sharp increase in effluent outlet temperature occurs.
When the price of natural gas price is high relative to crude, gas cracking tends to be disadvantaged when compared with the cracking of virgin crudes and/or condensates, or the distilled liquid products from those feeds (e.g., naphtha, kerosene, field natural gasoline, etc.). However, cracking heavier feeds, such as kerosenes and gas oils, produces large amounts of tar, which leads to rapid fouling in the transfer line exchangers preferred in lighter liquid cracking service, often requiring costly shutdowns for cleaning. Nevertheless, in such an economic environment, it would be desirable to extend the range of useful feedstocks to include liquid feedstocks that yield higher levels of tar. Therefore, there is a need for an improved process and apparatus for removing the resulting heavy oils and tars that foul transfer line exchangers, without the need for costly shutdowns.